Advances in radio frequency (RF) front end circuitry continue to provide improvements in signal quality and data throughput. One technique for improving signal quality and data throughput is by providing multiple antennas, which are used to simultaneously transmit and/or receive signals. FIG. 1 shows conventional RF front end circuitry 10 including a primary antenna 12A, a secondary antenna 12B, primary antenna switching circuitry 14A coupled to the primary antenna 12A, secondary antenna switching circuitry 14B coupled to the secondary antenna 12B, a first cross-coupling connection line 16A and a second cross-coupling line 16B coupled between the primary antenna switching circuitry 14A and the secondary antenna switching circuitry 14B, primary transceiver circuitry 18 coupled to the primary antenna switching circuitry 14A, and secondary receiver circuitry 20 coupled to the secondary antenna switching circuitry 14B.
The primary antenna switching circuitry 14A includes a primary antenna node 22, a primary transceiver node 24, a first cross-coupling connection node 26, and a second cross-coupling connection node 28. A first switch 30 is coupled between the primary antenna node 22 and the primary transceiver node 24. A second switch 32 is coupled between the primary transceiver node 24 and the first cross-coupling connection node 26. A third switch 34 is coupled between the primary antenna node 22 and the second cross-coupling connection node 28. The secondary antenna switching circuitry 14B includes a secondary antenna node 36, a secondary receiver node 38, a third cross-coupling connection node 40, and a fourth cross-coupling connection node 42. A fourth switch 44 is coupled between the secondary antenna node 36 and the secondary receiver node 38. A fifth switch 46 is coupled between the secondary receiver node 38 and the third cross-coupling connection node 40. A sixth switch 48 is coupled between the secondary antenna node 36 and the fourth cross-coupling connection node 42. The first cross-coupling connection line 16A is coupled between the first cross-coupling connection node 26 and the third cross-coupling connection node 40. The second cross-coupling connection line 16B is coupled between the second cross-coupling connection line 28 and the fourth cross-coupling connection line 42.
In a first mode of operation, the first switch 30 and the fourth switch 44 are closed, while the second switch 32, the third switch 34, the fifth switch 46, and the sixth switch 48 are open, thereby coupling the primary transceiver circuitry 18 to the primary antenna 12A and the secondary receiver circuitry 20 to the secondary antenna 12B. This configuration is illustrated in FIG. 2A. Accordingly, in the first mode of operation, primary RF transmit signals are provided from the primary transceiver circuitry 18 to the primary antenna 12A, primary RF receive signals are provided from the primary antenna 12A to the primary transceiver circuitry 18, and secondary RF receive signals are provided from the second antenna 12B to the secondary receiver circuitry 20. The secondary RF receive signals may be diversity multiple-input-multiple-output (MIMO) receive signals. In general, the first mode of operation is used when the performance of the primary antenna 12A is better than that of the secondary antenna 12B, for example, when the voltage standing wave ratio (VSWR) associated with the primary antenna 12A is lower than the VSWR associated with the secondary antenna 12B.
In a second mode of operation, the second switch 32, the third switch 34, the fifth switch 46, and the sixth switch 48 are closed, while the first switch 30 and the fourth switch 44 are open, thereby coupling the primary transceiver 18 to the secondary antenna 12B and the secondary receiver circuitry 20 to the primary antenna 12A. This configuration is illustrated in FIG. 2B. Specifically, the primary transceiver circuitry 18 is coupled to the secondary antenna 12B via the first cross-coupling connection line 16A, while the secondary receiver circuitry 20 is coupled to the primary antenna 12A via the second cross-coupling connection line 16B. Accordingly, in the second mode of operation, primary RF transmit signals are provided from the primary transceiver circuitry 18 to the secondary antenna 12B, primary RF receive signals are provided from the secondary antenna 12B to the primary transceiver circuitry 18, and secondary RF receive signals are provided from the primary antenna 12A to the secondary receiver circuitry 20. In general, the second mode of operation is used when the performance of the primary antenna 12A is worse than that of the secondary antenna 12B, for example, when the VSWR associated with the primary antenna 12A is higher than the VSWR associated with the secondary antenna 12B. Those skilled in the art will appreciate that the antenna swapping capability enabled by the conventional RF front end circuitry 10 allows the antenna with the best performance to be used for the primary transmission and reception of RF signals, which generally improves the signal quality of primary RF signals. Switch control circuitry 50 coupled to the first antenna switching circuitry 14A and the second antenna switching circuitry 14B may control the switches therein in order to switch between the first mode of operation and the second mode of operation.
Generally, the primary antenna 12A is provided at a first end of a mobile communications device, and the secondary antenna 12B is provided at a second end of the mobile communications device, which is opposite the first end. This is so that an obstruction (e.g., a user's hand, a surface on which the device is placed, etc.) at one end of a mobile communications device will not affect transmission and/or reception characteristics of both antennas 12 simultaneously, thereby preserving the performance of at least one of the antennas 12. FIG. 3 illustrates a mobile communications device 52 including the first antenna 12A, the second antenna 12B, the primary transceiver circuitry 18, and the secondary receiver circuitry 20. The primary antenna 12A is designed and placed in the mobile communications device 52 in order to be used for the primary transmission and reception of RF signals during normal operation. It is only when an obstruction of some kind limits the performance of the primary antenna 12A that the second mode of operation is used. Accordingly, the primary antenna 12A is used most of the time for the primary transmission and reception of RF signals, while the secondary antenna 12B is used most of the time for the reception of secondary RF signals. The conventional RF front end circuitry 10 is therefore designed to maximize performance during normal operation (i.e., the first mode of operation discussed above).
In order to maximize the performance of the conventional RF front end circuitry 10 in the first mode of operation, the distance between the primary antenna 12A and the primary transceiver circuitry 18 should be minimized. Similarly, the distance between the secondary antenna 12B and the secondary receiver circuitry 20 should be minimized. This is so that signals provided between the primary antenna 12A and the primary transceiver circuitry 18 and the secondary antenna 12B and the secondary receiver circuitry 20 experience minimal insertion loss and distortion that may be introduced by longer signal traces. Since the primary transceiver circuitry 18 and the secondary receiver circuitry 20 are on opposite ends of the mobile communications device 52 in such a configuration, the first cross-coupling connection line 16A and the second cross-coupling connection line 16B run the length of the mobile communications device 52 between the primary antenna switching circuitry 14A and the secondary antenna switching circuitry 14B to implement the antenna swapping capability discussed above. Generally, the first cross-coupling connection line 16A and the second cross-coupling connection line 16B are shielded lines (e.g., coaxial lines) in order to minimize insertion loss and interference. Because the second mode of operation is only temporarily used when the primary antenna 12A experiences a significant decline in performance, the decrease in performance due to the use of the cross-coupling connection lines 16 is considered an acceptable trade-off in order to increase the performance of the conventional RF front end circuitry 10 during normal operation.
In some situations, primary RF transmit signals provided at one of the antennas 12 may couple into the other one of the antennas 12, such that the secondary RF receive signals provided to the secondary receiver circuitry 20 include a portion of the primary RF transmit signals. Filtering circuitry may be provided between the secondary receiver circuitry 20 and the secondary antenna switching circuitry 14B to reduce the portion of the primary RF transmit signals coupled into the secondary receiver path, however, limits on insertion loss in the secondary receiver path may place design constraints on the filtering circuitry that limit the effectiveness thereof. Accordingly, there is a need for RF front end circuitry with improved primary RF transmit signal isolation in the secondary receive signal path.